Analysis of cross-stage bindings becomes convenient if we introduce the notion of metalevels.
Metalevel of a tree is a number that gets incremented every time you reify something and gets decremented when you splice something.
Metalevel of a symbol is equal to the metalevel of its definition.

Cross-stage bindings are introduced when symbol.metalevel != curr_metalevel.
Both bindings introduced in Example 1 are cross-stage.

Depending on what side of the inequality is greater, the following situations might occur:

1) symbol.metalevel < curr_metalevel. In this case reifier will generate a free variable
that captures both the name of the symbol (to be compiled successfully) and its value (to be run successfully).
For example, x in Example 1 will be reified as follows: Ident(newFreeVar("x", IntTpe, x))

2) symbol.metalevel > curr_metalevel. This leads to a metalevel breach that violates intuitive perception of splicing.
As defined in macro spec, splicing takes a tree and inserts it into another tree - as simple as that.
However, how exactly do we do that in the case of y.splice? In this very scenario we can use dataflow analysis and inline it,
but what if y were a var, and what if it were calculated randomly at runtime?

This question has a genuinely simple answer. Sure, we cannot resolve such splices statically (i.e. during macro expansion of reify),
but now we have runtime toolboxes, so noone stops us from picking up that reified tree and evaluating it at runtime
(in fact, this is something that Expr.splice does transparently).

This is akin to early vs late binding dilemma.
The prior is faster, plus, the latter (implemented with reflection) might not work because of visibility issues or might be not available on all platforms.
But the latter still has its uses, so I'm allowing metalevel breaches, but introducing the -Xlog-runtime-evals to log them.

upd. We no longer do that. In case of a runaway splice inside a reify, one will get a static error.
Why? Unfortunately, the cute idea of transparently converting between static and dynamic splices has failed.
1) Runtime eval that services dynamic splices requires scala-compiler.jar, which might not be on library classpath
2) Runtime eval incurs a severe performance penalty, so it'd better to be explicit about it

As we can see, the only problem is the fact that lhs'es of splice can be code blocks that can capture variables from the outside.
Code inside the lhs of an splice is not reified, while the code from the enclosing reify is.

Hence some bindings become cross-stage, which is not bad per se (in fact, some cross-stage bindings have sane semantics, as in the example above).
However this affects freevars, since they are delicate inter-dimensional beings that refer to both current and next planes of existence.
When splicing tears the fabric of the reality apart, some freevars have to go single-dimensional to retain their sanity.

Example 2. Consider the following snippet:

reify {
val x = 2
reify{x}.splice
}

Since the result of the inner reify is wrapped in a splice, it won't be reified
together with the other parts of the outer reify, but will be inserted into that result verbatim.

The inner reify produces an Expr[Int] that wraps Ident(freeVar("x", IntTpe, x)).
However the freevar the reification points to will vanish when the compiler processes the outer reify.
That's why we need to replace that freevar with a regular symbol that will point to reified x.

Example 3. Consider the following fragment:

reify {
val x = 2
val y = reify{x}
y.splice
}

In this case the inner reify doesn't appear next to splice, so it will be reified together with x.
This means that no special processing is needed here.

split the list into groups where every placeholder is always
put in a group of it's own and all subsquent non-holeMap are
grouped together; element is considered to be a placeholder if it's
in the domain of the fill function;

2. fold the groups into a sequence of lists added together with ++ using
fill reification for holeMap and fallback reification for non-holeMap.

When untangling reifier symbol tables from the reifier itself,
I discovered that encoding of a symbol table (e.g. producing corresponding reificode)
might cause subsequent reification (e.g. when filling in signatures and annotations for syms).

This is a mess in the face of nested reifications, splices and inlining of thereof,
so I made SymbolTable immutable, which brought a significant amount of sanity.

However that wasn't enough. Sure, symbol table became immutable, but the reifier still needed
to mutate its symtab field during reification. This caused nasty desyncs between the table being encoded
and the table of the underlying reifier, so I decided to encapsulate the entire state here,
so that encoding can backup the state before it starts and restore it after it completes.

Keeps track of auxiliary symbols that are necessary for this reification session.
These include:
1) Free vars (terms, types and existentials),
2) Non-locatable symbols (sometimes, e.g. for RefinedTypes, we need to reify these; to do that we create their copies in the reificode)
3) Non-locatable symbols that are referred by #1, #2 and #3

Exposes three main methods:
1) syms that lists symbols belonging to the table,
2) symXXX family of methods that provide information about the symbols in the table,
3) encode that renders the table into a list of trees (recursively populating #3 and setting up initialization code for #1, #2 and #3)